1. Field of the Invention
The present invention relates to illumination systems and more particularly to exterior high intensity discharge (HID) illumination systems which are responsive to changes in time of day and in ambient light.
2. Brief Description of the Prior Art
Conventionally, dimming of outdoor HID light fixtures such as those used in street lights and for parking lots and loading docks is performed by a dimming unit mounted in an enclosure separate from the fixture. Typically such enclosures are located indoors. These dimming units are designed to control several lights simultaneously.
There are two techniques commonly used to dim HID light sources. The first makes use of a solid-state switch such as a triac or silicon controlled rectifier and dims the lights by turning the power on and off 120 times a second. Power flow is varied by controlling the amount of time the power is on versus the amount of time the power is off. The longer the power is left off during each half cycle, the more the lights are dimmed. This control method is commonly referred to as phase control. The second technique requires the use of an electronic dimming ballast that allows the ballast to be internally addressed to vary lamp arc current and, hence, the lamp's light output.
On/off control for single outdoor HID fixtures is frequently performed by a photocontrol unit mounted atop the fixture. The photocontrol units sense ambient light level and thereby turn the light on at dusk and off at dawn. A particular prior art photocontrol unit uses a timer to allow the user to predetermine when the light should turn on and off each night i.e., permits the light to be turned on and off at times other than dusk and dawn. Prior art photocontrol units have been capable of on/off control only but have not allowed for dimming.
One disadvantage of the prior art HID light dimming products is a lack of flexibility. Prior art dimming units are conventionally designed to dim several lights at once. There are many applications where it is desired that only a small portion of the lights in a space be dimmed or that the lights be dimmed to different levels. Using such conventional units for these applications would require extensive rewiring of the lighting circuit, and hence make it difficult to make future changes. Also, the control circuitry in the prior art dimming units requires a continuous supply of electrical power. Hence, such dimming units are difficult use in applications where power is not accessible during the day. For example, the power fed into some street light circuits is controlled by a single photocontrol unit, and during the day power is not available at the light fixture. Another disadvantage to the prior art units is their size. The size of the existing dimming units usually prevents them from being mounted directly to the fixture where wiring is more readily accessible for installation and maintenance. Finally the prior art units are often costly. The electrical capacity of the existing dimming units is generally sized to provide dimming for several light fixtures simultaneously. The extra cost required to provide this extra capacity makes these units impractical for dimming single fixtures. Also, for dimming units that require a dimming ballast to be used, there is a considerable cost associated with replacing the existing lamp ballasts, making these units impractical in most retrofit applications.
It is, therefore, the object of the present invention to provide an outdoor HID lighting system which can be dimmed in a simple, cost effective manner.